Narrative:

Uneventful takeoff and initial climb out. The first officer (first officer) was the pilot flying and I was the pilot monitoring. At approximately 16;000 ft.; I received a call from one of the flight attendant's (flight attendant) in the aft of the aircraft informing me that they had a loud high pitched squeal coming from [what] they thought [was] one of the aft cabin doors. They suspected a door seal leak. They also informed me that they had had some smoke and odor from one of the aft ovens and had turned the ovens off. I advised them to wait a few minutes and see if the squeal went away as we pressurized and call me back if it got worse or didn't improve. A few minutes later; they called again to inform me that it was not a door seal leak but rather the smoke alarm in the aft lavatory that was going off. They informed me that there was no visible smoke at this point but that there was a burnt plastic and food odor. I verified with the flight attendant's that the ovens were off. We also were beginning to smell a faint burning smell in the cockpit as well. The first officer and I went on O2 (oxygen) and I went through the smoke; fire and fumes checklist while the first officer continued to fly the airplane and handle the radios. As per the checklist; I directed the flight attendant's to pull the circuit breakers for the ovens in the aft galley. The smoke alarm continued to go off and according to the flight attendant's; there was no smoke visible in the area but there was still a burning plastic/food odor. At this point I decided the best course of action was to return to ZZZ. I directed the first officer to advise ATC and [request priority handling] and start us back to ZZZ. I briefed the purser that we would be returning to ZZZ and to go ahead and prepare for a possible evacuation upon arrival as well as the approximate time until touchdown. I also asked them to keep me apprised of any changes regarding the situation in the aft galley. We were given initial vectors turning us back to ZZZ and then direct to zzzzz intersection to join the arrival into ZZZ for the ILS to xxr. I proceeded to brief the passengers on the nature of the emergency and that while we were prepared for a possible evacuation I did not think that it would be necessary but to listen for directions from the cockpit or the flight attendant's once we landed. I also updated the flight attendant's on expected touchdown and checked to make sure that they were okay and that they were all set in the back.we had an uneventful approach and landing into ZZZ; touching down. We landed approximately 15;000 lbs. Overweight with a soft touchdown. Station fire department personnel and vehicles followed us to the gate and boarded the aircraft. They checked the ovens; aft galley; and lavatories and gave the all clear. The passengers were then given the okay to deplane. Maintenance write-ups were done for the smoke and fumes as well as the overweight landing. The [chief pilot] was called and updated on the event.

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Original NASA ASRS Text

Title: B757 Captain reported being notified of a lavatory smoke alarm going off and detected a faint burning odor in cockpit; resulting in a return to the departure airport.

Narrative: Uneventful takeoff and initial climb out. The FO (First Officer) was the Pilot Flying and I was the Pilot Monitoring. At approximately 16;000 ft.; I received a call from one of the FA's (Flight Attendant) in the aft of the aircraft informing me that they had a loud high pitched squeal coming from [what] they thought [was] one of the aft cabin doors. They suspected a door seal leak. They also informed me that they had had some smoke and odor from one of the aft ovens and had turned the ovens off. I advised them to wait a few minutes and see if the squeal went away as we pressurized and call me back if it got worse or didn't improve. A few minutes later; they called again to inform me that it was not a door seal leak but rather the smoke alarm in the aft lavatory that was going off. They informed me that there was no visible smoke at this point but that there was a burnt plastic and food odor. I verified with the FA's that the ovens were off. We also were beginning to smell a faint burning smell in the cockpit as well. The FO and I went on O2 (Oxygen) and I went through the Smoke; Fire and Fumes checklist while the FO continued to fly the airplane and handle the radios. As per the checklist; I directed the FA's to pull the circuit breakers for the ovens in the aft galley. The smoke alarm continued to go off and according to the FA's; there was no smoke visible in the area but there was still a burning plastic/food odor. At this point I decided the best course of action was to return to ZZZ. I directed the FO to advise ATC and [request priority handling] and start us back to ZZZ. I briefed the Purser that we would be returning to ZZZ and to go ahead and prepare for a possible evacuation upon arrival as well as the approximate time until touchdown. I also asked them to keep me apprised of any changes regarding the situation in the aft galley. We were given initial vectors turning us back to ZZZ and then direct to ZZZZZ intersection to join the arrival into ZZZ for the ILS to XXR. I proceeded to brief the passengers on the nature of the emergency and that while we were prepared for a possible evacuation I did not think that it would be necessary but to listen for directions from the cockpit or the FA's once we landed. I also updated the FA's on expected touchdown and checked to make sure that they were okay and that they were all set in the back.We had an uneventful approach and landing into ZZZ; touching down. We landed approximately 15;000 lbs. overweight with a soft touchdown. Station fire department personnel and vehicles followed us to the gate and boarded the aircraft. They checked the ovens; aft galley; and lavatories and gave the all clear. The passengers were then given the okay to deplane. Maintenance write-ups were done for the smoke and fumes as well as the overweight landing. The [Chief Pilot] was called and updated on the event.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.