Date: 201902
Local time of day: 1201-1800
Location: ZZZ.Airport, US
Flight conditions: VMC
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class B ZZZ
ATC / advisory: Tower ZZZ
Aircraft component: Door
Component problem: Improperly Operated; Malfunctioning
Reporter organization: Air Carrier
Reporter function: Captain; Pilot Flying
Reporter qualification: Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Human factors: Troubleshooting
Second reporter function: Pilot Not Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Ground Event / Encounter Other / Unknown
Detector: Person Flight Attendant
When detected: In-flight
Result: Flight Crew Returned To Departure Airport; Flight Crew Landed in Emergency Condition
Contributing factors: Human Factors; Procedure
Primary problem: Human Factors

Narrative: We departed ZZZ on schedule. Taking off on RWY XXR from intersection W with an approximate gross weight of 172;000 lbs. Climb out and clean up were normal. Climbing through about 8500 feet; we received a call from a Flight Attendant informing us that the handle on the left aft cabin door was moving about freely. We verified that the aircraft was pressurizing properly. We discontinued our climb; topping out at about 13;000 feet and leveling off initially at 11;000 feet and then proceeding below 10;000 feet. I sent First Officer back to investigate; and he reported that the handle was fluttering back and forth rapidly.We elected to return to ZZZ; [advising ATC] due to the overweight landing. We requested and received delay vectors. We performed a landing assessment; completed the Overweight Landing Checklist and the Before Landing Checklist. After preparing for an ILS and landing on RW XXR; we proceeded to return to the airport.I flew a normal ILS with a 30 flap Vref of 159 knots. This provided a stable descent rate of just over 900 fpm on glideslope. Between 100 and 50 feet above touchdown; we picked up some additional sink. I attempted to arrest the sink with power and some additional pitch; but still touched down firmly at about 900 fpm.Roll out and taxi were normal. After shutdown my discussions with Maintenance; personnel suggested that there may be indications of aircraft over stress. The actual overweight landing inspection was still pending at that point.It was determined that the external handle on the left aft cabin door had not been properly stowed. This was the sole and completely avoidable reason for our return to ZZZ.Even though we were below the maximum overweight landing limit for this aircraft; I could have elected to remain airborne longer to burn more fuel. This would have provided a greater margin of error for the landing flare.


Second narrative: Shortly after takeoff; the rear Flight Attendant called the cockpit and said the aft; left side service door handle was flopping in the wind. The handle wasn't secured correctly.  We [advised ATC]; did the necessarily checklists and returned to ZZZ. We landed overweight in ZZZ; we were assigned a new plane and continued onto [destination].


Synopsis: Air carrier flight crew reported a door handle anomaly resulted in a return to the departure airport.