Ben 10 as a franchise is filled to the brim with memorable story arcs. However, I don't think the series always necessarily delivers them in the best way possible. That's mostly because of the format. Ben 10 is an episodic TV show, so usually episodes are split between adding to whatever ongoing story arc is going on and one-off or otherwise episodic adventures. While there's nothing inherently wrong with that, it does mean that the series' story arcs are padded with filler in the middle, which I feel really hurts them. So I've made a list with just the story episodes for these arcs. Mind you, this is not an assessment of what Ben 10 episodes are important to watch as a whole to understand the series, just ones that are pertinent to these story arcs. If you want to watch the series for the first time and get into it, a lot of episodic adventures are very important.

That being said, not all of these are so cut-and-dry to nail down. There are a few episodes that are in a bit of a "grey area". I'll add observartions if I feel they're needed. Generally, I'd like you to be mindful of the concept that there are episodes which are not part of the main storyline, but that use said storyline's events as a plot point. Think "Game Over", which takes place during the Ghostfreak Arc, and whose inciting incident for the plot is the purple lightning being generated by Ghostfreak's lackeys. There is a one-way relationship between the story arc there. Generally, understand an "*" to mean the episode mentioned is skippable, to your discretion. And finally, if there's a story arc you don't particularly enjoy, especially in Ultimate Alien and Omniverse, it might have been because of the filler in-between important episodes. I'd recommend watching them how I layed them out here. I think there's hardly any genuinely bad story arcs**. You might find the enjoyment in them by experiencing them as a serialized story!**

The Vilgax Arc (Original Series)

1. And Then There Were Ten
2. Hunted
3. The Alliance
4. Secrets

The Kevin 11 Arc (Original Series)

1. Kevin 11
2. Framed
3. Grudge Match
4. Back with a Vengeance

*Considering Cannonbolt's spotlight at the end of "Grudge Match", you might want to add "The Big Tick" to the list. Other than that, it's not relevant to this storyline.

The Ghostfreak Arc (Original Series)

1. Last Laugh*
2. Ghostfreaked out
3. Benwolf
4. Under Wraps
5. The Return
6. Be Afraid of the Dark

*Although skippable here, the episode "Last Laugh" is a strongly recommended addition to start off the arc. At the ending, we have the biggest moment that hints towards Ghostfreak's true nature. Additionally, many events of this episode are directly followed up on by "Ghostfreaked Out".

**As mentioned in my introduction, there are self-contained episodes related to this story arc. Those would be "Game Over" and "Monster Weather". Neither of those contains anything important for this story arc and are skippable for its purposes, but the purple lightning created by Ghostfreak's lackey is responsible for the events of these episodes.

The Highbreed Invasion Arc (Alien Force)

*I'll just preface this by saying, I think all 26 episodes from "Ben 10 Returns" to "War of the Worlds" genuinely are worth watching for this story arc. All episodes add in ways big and small to the overarching story, and the journey of them all culminates very well into the finale. I think that's largely the reason the Highbreed arc is so fondly remembered. So I consider all 26 story episodes. However, if you really want to skip some episodes (which again, I advise against), I'll mark the least important ones for the overarching story with a "*".

1. Ben 10 Returns Part 1
2. Ben 10 Returns Part 2
3. Everybody Talks About the Weather
4. Kevin's Big Score
5. All That Glitters
6. Pier Pressure
7. Max Out
8. Be-Knighted*
9. What Are Little Girls Made Of?
10. The Gauntlet*
11. Plumbers' Helpers
12. Paradox
13. X = Ben + 2*
14. Alone Together
15. Darkstar Rising
16. Good Copy, Bad Copy*
17. Save the Last Dance
18. Undercover
19. Birds of a Feather*
20. Grounded*
21. Pet Project
22. Unearthed*
23. Voided
24. Inside Man
25. War of the Worlds Part 1
26. War of the Worlds Part 2

The Vengeance of Vilgax Arc (Alien Force)

*This season is actually a bit fast and loose with story, so I'll do my best to narrow down the main plot here. I consider story episodes in this season the ones that deal with Vilgax's offensive against Ben, and Kevin's mutation. I use that because those are the main conflicts set up in the premiere, and the ones resolved in the ending.

1. Vengeance of Vilgax Part 1
2. Vengeance of Vilgax Part 2
3. In Charm's Way
4. Trade Off
5. Ghost Town
6. Primus
7. The Secret of Chromastone
8. Time Heals
9. The Final Battle Part 1
10. The Final Battle Part 2

The Agreggor / Ultimate Kevin Arc (Ultimate Alien)

1. Fame
2. Escape from Aggregor
3. Too Hot to Handle
4. Andreas' Fault
5. Fused
6. Ultimate Aggregor
7. Map of Infinity
8. Deep
9. Where the Magic Happens
10. Perplexahedron
11. The Forge of Creation
12. ...Nor Iron Bars a Cage
13. The Enemy of My Enemy
14. Absolute Power Part 1
15. Absolute Power Part 2

The Dagon Arc (Ultimate Alien)

1. The Creature from Beyond
2. Prisoner Number 775 is Missing
3. The Purge
4. The Flame Keepers' Circle
5. A Knight to Remember
6. Solitary Alignment
7. The Ultimate Sacrifice**
8. The Beginning of the End
9. The Ultimate Enemy Part 1
10. The Ultimate Enemy Part 2

*The episode "Enemy of My Frenemy" benefits from this arc by having Dagon facilitate the climax, but its story doesn't necessarily add to it. It's in the same category as "Game Over"

**I would personally recommend watching "The Ultimate Sacrifice" alongside this arc where I placed it. Although on a story level it doesn't factor into the arc, it's thematically very important. Questions of Ben's "worthiness" of being the wielder of the Omnitrix, and his bonds with his friends are a major plot point in the finale, and this is the episode that deals with those questions. I do feel, however, the production order places it very awkwardly for these purposes. The story takes a "break" after "Solitary Alignment", and I think that's where an unrelated but thematically important episode would fit more naturally

The Malware / Feedback Arc (Omniverse)

1. The More Things Change Part 1
2. The More Things Change Part 2
3. It Was Them
4. Trouble Helix
5. Of Predators and Prey Part 1
6. Of Predators and Prey Part 2
7. Outbreak
8. Malefactor
9. Showdown Part 1
10. Showdown Part 2

*For this arc, I consider story episodes the ones that deal with Dr. Psychobos, Malware, and Khyber's offensive against Ben 10. The episode "A Jolt from the Past" features a very brief appearance by Feedback, and the episode "Many Happy Returns" are Gwen and Kevin's formal reintroduction to the series, and although they show up in the finale, I think it's unimportant since they only show up in the finale and the context of this episode is largely irrelevant for that, especially since they also show up at the beginning of the arc as well. Ultimately, I think neither of these episodes justify their addition.

The Incursean Arc (Omniverse)

1. Bros in Space*
2. Tummy Trouble
3. Vilgax Must Croak
4. While You Were Away
5. The Frogs of War Part 1
6. The Frogs of War Part 2

*I think "Bros in Space" is a worthwhile addition to watch alongside this arc due to the events of "While You Were Away" directly following up on many of its plot points.

**The episode "Rad" benefits from this arc by having the Incursean offensive as a background, but its events have no impact on the overarching story, and don't feature its main antagonists.

The Albedo Arc (Omniverse)

1. Double of Nothing (Ultimate Alien)*
2. The Ultimate Heist
3. A Fistful of Brains
4. For a Few Brains More

*The Ultimate Alien episode "Double of Nothing" has many of its plotpoints directly followed up on by this arc. It think it's worth a watch alongside it.

The Galactic Monsters Arc (Omniverse)

1. Mystery, Incorporeal**
2. Rad Monster Party
3. Charmed, I'm Sure
4. The Vampire Strikes Back

*The "Galactic Monsters" initiative was more indicative of the general "vibe" the fifth season of Omniverse was going to have, focused on Ben 10's more spooky Halloween-y side. I consider the story arc of this season the plot involving Ghostfreak and the Anur System.

**"Mystery Incorporeal" is largely irrelevant in a plot sense, but it marks both Ghostfreak's (both the transformation and the villain) and Charmcaster's re-introduction into the series. Both are major characters for this storyline.

The Rooters Arc (Omniverse)

1. Max's Monster*
2. The Rooters of All Evil
3. Universe v. Tennyson**
4. Weapon XI Part 1
5. Weapon XI Part 2

*"Max's Monster" I think is a worthwhile addition for being centered around Phil's and Max's relationship. While Phil is largely a secondary character to this arc, all of the interactions he does have with other characters, especially Max, as well as his powerset, are informed by the events of this episode.
**For "Universe v. Tennyson", Ben's control over Alien X is very important for a major scene in "Weapon XI", as well as his recreation of the Universe and full control of a Celestialsapien being strong points for Servantis' arguments. I do think that's intentional, as his speech is set to the background of Ben utilizing Alien X to fight.

The Vengers Arc (Multi-Series)

Just like any good connected universe, this arc spans isolated installments from different series, before culminating in the ultimate team-up episode. Also, while Captain Nemesis does have a second episode in "To Catch a Falling Star", the events of that episode are completely ignored, and arguably contradicted by his appearance in "The Vengers". Just don't watch it for this storyline. Those two don't fit.

1. Super Alien Hero Buddy Adventures (Original Series)
2. Hero Time (Ultimate Alien)
3. Arrested Development (Omniverse)
4. The Vengers (Omniverse)

The Time War Arc (Omniverse)

For this arc, I consider story episodes the ones that feature Professor Paradox, Chrono Spanner and/or Maltruant, and to a less rigid degree, Skurd.

1. So Long, and Thanks for All the Smoothies*
2. Special Delivery**
3. Ben Again
4. An American Benwolf in London***
5. Animo Crackers
6. And Then There Were None
7. And Then There Was Ben
8. Cought It Up
9. Fight at The Museum
10. It's a Mad, Mad, Mad Ben World Part 1
11. It's a Mad, Mad, Mad Ben World Part 2
12. Stuck on You
13. Let's Do the Time War Again
14. The Secret of Dos Santos
15. The End of an Era
16. A New Dawn

*While not to do with any of the parameters I use, "So Long, and Thanks for All the Smoothies" is the introduction to the Annihilarg, and the Contumelia, which is payed off at the end of the arc. This an extremely strongly recommended.
**Similarly, but to a lesser degreee, "Special Delivery" is the introduction to another device that becomes important at the conclusion of the story. I would also watch it before "Ben Again", just because it's an otherwise awakward stop in the story. I think it watches better before.

***"An American Benwolf in London" is Kai Green's proper re-introduction to the series, and since she is a major character for this arc, it's a recommended watch alongside the others. The events of this episode are largely irrelevant outside of that, for the exception of a small payoff when we get to see Future Kai.

****The episodes "Malgax Attacks" and "The Most Dangerous" episodes include incredibly minor plotpoints for Ben's relationships to Skurd and to Kai, respectively. Nothing remotely justifiable for a rewatch. And also, those episodes suck. Don't watch them.

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I will also put below a list of episodes you might want to rewatch before rewatching the Time War Arc. That would be because some of the Time War episodes make strong callbacks to those. And we get to see a variety of characters from other timelines and futures. The ones I recommend are:

1. And Then There Were Ten (Original Series)
2. Gwen 10 (Original Series)
3. Ben 10,000 (Original Series)
4. Ken 10 (Original Series)
5. Ben 10,000 Returns (Ultimate Alien)
6. Ben 23 (Omniverse)

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