B737-800 Captain reported an air turn back after the L1 Door Operating Handle would not stay in the closed and locked position. An overweight landing was performed at departure airport.

Narrative

On Day 1 I was acting as PIC and Pilot Monitoring (PM) of Aircraft X from ZZZ to ZZZ1. This flight resulted in a return to the field for an issue with the main cabin door handle. Aircraft X was originally scheduled to depart ZZZ at XD59 local time. At about XA30 local time; I noticed that the flight was delayed for maintenance until XJ00. I confirmed the delay with the Dispatcher assigned our flight and then contacted crew tracking to remain at the hotel until the new departure time. The crew reported at the new time and began pre-flight preparations. As part of my Flight Attendant (FA) brief; I reminded the FA's to double check the door handle. Pre-flight proceeded smoothly and the flight pushed back at XI59 local. As we taxied out; there were no abnormal indications and we took off at XJ14 local. Our take off and initial climb proceeded with no abnormal indications. We departed via the ZZZZZ departure with the ZZZZZ transition. We checked in with ZZZ Departure; and were instructed to proceed direct ZZZ2 once passing 10;000 ft.; and to maintain 11;000 ft.. As we were nearing 10;000 ft.; the First Officer (FO); who was acting as Pilot Flying (PF); loaded direct ZZZ2 in the FMC and I verified it. As we were passing 10;000 ft. the FO executed the FMC. As I was reaching for the chime switch for the 10;000 foot chime; the number one FA called to notify me of the door handle issue. She stated that the main cabin door handle was moving upward and would not stay in the down and locked position. She further stated that they had to hold it down to keep it from moving; and that the force needed to hold it was increasing as we continued to climb. The FAs later stated that they were actually sitting on the handle and it was still moving. I acknowledged the issue and made the decision that we would return to the field. I instructed them to do whatever was necessary to hold the handle down and that I would get right back to them with more information. At this point; I informed ZZZ Departure of the issue; requested a decent and vector back to ZZZ. I then gave the FO the radios and asked him to request the trucks standing by to check the brake temperatures and to set up for an approach while I coordinated with the Company and the FAs. I sent dispatch an ACARS; called Operations to request a Gate. I called the FAs and briefed them on the return and then made a PA for the passengers. I then ran the overweight landing checklist and non-routine landing checklist. I calculated our landing weight as 158;200 lbs and our landing assessment was about 4;700 ft. Runway XXL is 14;511 ft.; so there was plenty of Runway for a flaps 30 landing. The FO and I discussed the overweight landing during the approach briefing and he elected to land with Auto Brakes off to minimize the potential for hot brakes. I agreed since it was a long dry Runway. As I was finishing all the QRH checklists; we were on a vector to final; just about to join the localizer. I ran the decent flow followed by the before landing checklist. We checked in with the Tower and were cleared to land. Touchdown was slightly firm with a decent rate of 350 fpm. On rollout; the FO deployed the thrust reversers; but let the aircraft roll without brakes. At about 60 kts.; He stowed the reversers as I called my aircraft and slowly started braking and came to a full stop on the Runway at the PA intersection; I made a PA asking the passengers to remain seated and reminding them that they will see safety vehicles. Our brakes were inspected and found to be within normal temperatures; and we requested that the trucks follow us to the Gate. We taxied to the Gate without further incident. The event was only related to the main cabin door handle. Since I believed this was mechanical; I don't know what could prevent this from occurring in the future.