Can someone explain the Burt and Irving plot in S2 E9?

I feel like I missed an episode or something because how did we get from dinner at Burts to Burt being a Lumon thug and Irving boarding a train? What am I missing? In episode 7 (I think) we saw what looked like Ricken going through Irving’s chest. Next thing we know Burt is sitting in Irving’s house reading his notes. I’m very confused.