Raiders, I think, has a three part structure. The intro of Indie and the setup with the villain in the South American temple. The Search for and discovery of the Arc, and the chase for and opening of the arc. So...
1. The Idol
2. Indy finds the Arc. The Arc is stolen.
3. Indy gets the Arc back from the Nazis.
Actually, despite its "serial" roots, Raiders follows the conventions of the Three-Act structure fairly closely.
First Act: Introduction
*Introduce Indy the daredevil archeologist
*Introduce Belloq, his rival who states "there is nothing you can possess that I cannot take"
*Introduce Indy's fear of snakes
*Introduce the goal: get the Ark
*Introduce the obstacle: Nazis
Second Act; Complications
*Indy followed by Nazis
*Key item (headpiece) held by bitter former flame Marion
*Nazi Toth arrives and approaches/theatens Marion
*Indy battles Toth's thugs. Toth burns hand on headpiece. Indy and Marion escape with headpiece
*Sallah tells Indy that Nazi's have a headpiece and are digging at Tanis
*Indy learns Nazis have only one side of headpiece (from Toth's hand) and are "digging in the wrong place"
Midpoint Twist
*Marion appears to be killed
*Belloq appears and suggests the Ark is a "radio for talking to God"
*Indy gets into map room at Tanis, uses headpiece to find actual Ark location
*Indy discovers Marion alive, held by Belloq
*Indy leaves Marion because her escape would put his getting the Ark at risk
*Toth reappears: reveal Nazi headpiece based the pattern burned into his hand
*In order to get to Ark, Indy must face his big fear: snakes
*Marion tries to escape, fails
*Indy gets Ark, Indy loses Ark immediately to Belloq, but gets Marion
*Indy and Marion escape death trap
*Indy intercepts truck and gets Ark back
*Indy on boat loses Ark and Marion to Nazis and Belloq
Third Act: Resolution
*Indy confronts Belloq and threatens to blow up the Ark
*Belloq calls his bluff
*Belloq opens the Ark to discover its secrets, but finds it just filled with sand
*TWIST: The Ark IS the real thing and Belloq and the Nazis are destroyed for daring to look within
*Indy & Marion Survive because they turn away as Indy knows the story of the Ark and understands that to look is to die
*Indy delivers the Ark to the government and gets rewarded, but...
*2nd TWIST: The Ark "lost", again, placed in an anonymous crate in an endless warehouse of crates
In fact, I never made the connection that the "lost ark" gets "lost" again in a figurative sense at the end of the movie until I typed up this breakdown!