Sam Loomis (John Gavin)’s Timeline and Summary

Sam divorces his wife; the alimony payments mean he's deeply in debt.
As a result, he can't marry his new girlfriend Marion. She's unhappy about this state of affairs, and tells him so during one of their lunch-break trysts.
Some days later, Lila, Marion's sister arrives at his business and tells him Marion has disappeared.
Detective Arbogast shows up and tells Sam that Marion is believed to have stolen some money.
Arbogast agrees to work with Lila and Sam. After some investigation, he calls them and tells them he's traced Marion to the Bates Motel.
Arbogast then disappears. Lila convinces Sam that the two of them need to act.
Sam goes to try to find Arbogast at the Bates Motel, but he isn't there.
Sam and Lila contact the local sheriff, who does nothing. So Lila and Sam go to the Bates Motel, pretending to be man and wife.
Sam distracts Norman while Lila searches the house.
Norman gets suspicious and knocks Sam out.
But Sam wakes up in time to prevent Norman, in his mother's clothes, from killing Lila in the house.
Sam and Lila listen to the psychiatrist explain Norman's pathology.
This is where Sam learns that Marion was killed, though you don't get to see his reaction, really.
Presumably he's upset, though.