In a nutshell (as far as I remember):

A rift in space-time which may or may not have been caused by the dragon Veldora (who was simply bored) currently being de-sealed inside Rimuru has sweeped up Rimuru and the girls transporting them to Spring Garden and separating them as well.
Rimuru in his slime form has been noticed by Tiger Lily and reported back, organizing a search party for the mysterious organism.
During their search, Commander with his squad encountered Shion, Treyni and Millim during a fight with pests, looking for Rimuru as well.
As it turns out, the guests received a blessing from the World Flower, giving them a flower name each.
They decided to cooperate since their objective was the same.
Following sightings of Rimuru they follow his trail and eventually reunite during a battle with pests.
Shizu also joined the battle, which was a surprise for Rimuru, since she's already dead in the world they came from (since it was a space-time rift).
Finding him, the group returns to the town to exchange intel and grab a bite.
The flower knights noticed the boss pest can create portals mentioning it as a possibility for the guests to return home, to which Rimuru responded he can absorb the pest to gain its abilities once it's weak enough.
The group seeks out the boss pest again and thanks to everybody's cooperation the plan ends with a success.
Following the ol' Asterix and Obelix plot pattern, they all return for a final feast to the town.

As for the character quests, they made a cheap move and the first two for every collab girl are the same - a feast in town with the flower girls, in which Shion tries to help with food preparations (which is a running gag with her since she's an awful cook), Millim trying to keep her in check, Treyni stuffing her cheeks since she's a bit of a gourmet even while being a dryad and Shizu enjoying the party from afar.
The last episode is when Commander goes out of the restaurant to enjoy some fresh air and encounters the collab girl. They spend some time together sharing their thoughts about the experiences they encountered in Spring Garden and the guests' world.