Basically, Francis came home because he wanted to steal a fish that he caught so he could impress a 12 year old that lived in his neighborhood. Hal said that it was his fish, but Francis is the one who actually caught it because he spent hours reeling it in. Francis didn't want anybody to know that he was there so he was sneaking around outside where he ran into Jamie, who was holding one of those metallic balloons. Jamie got excited to see Francis and let go of the balloon, which floated up and shorted out the power box causing a blackout.
Francis runs into Lois and begs her not to tell Hal that he's home because he's having marital problems. (He made up a story about his marriage failing because he doesn't want her to know that he's really there to steal the fish.) So Lois tries to keep Hal and Francis apart in between discussing is "marital problems" with him.
It's Hal and Lois's anniversary and Hal was orginally going to take her out to dinner, but he changed plans when he came into possession of 2 very expensive cuts of steak (kobe beef). He covinces Reese to cook the steaks for them by telling him that he will get one of the good steaks, but he really plans on switching them at the end and giving the good one to Lois.
Meanwhile, Malcolm is trying to sneak 3 swedish girls into the house because he thought that Hal and Lois were going to be gone for the evening. Plans changed when they decided to stay home so he had to figure out how to get Hal and Lois out of the house so he and Reese could "enjoy the girls' company" in private. Reese wants to drug them so they'll go to sleep, but Malcolm decides to call and tell them he's drunk at a party an hour away. Hal and Lois were just about leave and go get him when the lights came back on and they found out that Malcolm and the girls were in the house.
Reese put the drugs in the steak sauce anyway. He was only going to put it on Lois and Hal's steaks, but Dewey came along and put it on all the steaks--causing the whole family to pass out in the middle of their meal. At the beginning of the episode Hal had told Dewey that he could eat cereal for dinner, but then said he couldn't because they were having steak. The final scene shows Dewey getting ready to eat cereal as he sits down between the three Swedish girls on the couch--while the rest of the family is passed out in the kitchen.
It was a great episode! There's more about it in the episode discussion thread at
http://www.malcolminthemiddle.co.uk/forum/showthread.php?p=11299#post11299
Nice summary Christina. The only correction I have is that the girls are Dutch, not Swedish.