They must be trying to return the stolen plans to the princess.
6 seconds sound clip from the Star Wars: Episode IV - A New Hope (1977) movie soundboard.
You can hear this line at 01:06:08 in the Blu-ray version of the movie.
Quote context
[...]
- We are opening the magnetic field.
- To your stations! Come with me.
- Yes?
- We've captured a freighter entering the remains of the Alderaan system.
- Its markings match those of a ship that blasted its way out of Mos Eisley.
- They must be trying to return the stolen plans to the princess.
- She may yet be of some use to us.
- Unlock 1, 5, 7, and 9. Release charge.
- There's no one on board, sir. According to the log, the crew abandoned ship right after takeoff.
- It must be a decoy, sir. Several of the escape pods have been jettisoned.
[...]