While oil and gas assets are gaining more maturity, plug and abandonment (P&A) operations have drawn much attention from the industry, presenting strong economic challenges. When a severe integrity issue drives the necessity to abandon a subsea well while also preventing the safe installation of a blowout preventer (BOP), the challenges are far beyond usual ones. One of the most complex P&A operations, which requires the commitment and expertise of the operator and all services providers, involves coiled-tubing (CT) and wireline services. In 2018, after an operator transfer, a 3-year-old well that had been the best field producer was found to have compromised foundations and the tree abnormally tilted by ±5°. An urgent P&A was required, and this was planned in two phases: Phase 1, in 2019, consisted of temporarily securing the well to allow time for the P&A design. This was done by bullheading cement directly from a drillship in the completion through a specifically designed light pumping cap, set on top of the tree, which was a challenging operation.Phase 2 consisted of a compliant and definitive abandonment, which was feasible only after having verticalized and strengthened the well and tree with tethers. The P&A was successfully achieved in 2021, after a 6-month heavy workover, which mixed through-tubing intervention for the temporary abandonment of the well and open-water operations (for completion retrieval and B-annulus cement requalification, before placement of definitive barriers in production casing). This study describes the challenges of the through-tubing operations, which included the following: Recover deep access to the well to temporarily abandon it, in a control-line-free area, located below the Phase 1 in-tubing cement plug.The biggest uncertainty was relying on the questionable length of this cement plug. A latest-generation intelligent CT with hybrid telemetry system, providing reliable downhole data was selected to safely mill the in-tubing cement while monitoring the "hole" cleaning and guarantee the accurate positioning of suspension barriers in a 67-m-high control-lines-free A-annulus window above seal bore packer.Perform complex placement and qualification of the cement plug in the A-annulus with CT before open-water operations, which required the combined effort of the CT and wireline companies.Perform remediation of a hydrate plug, below the tubing hanger. This remediation was a strong requirement to allow circulating the A-annulus to place temporary barriers, and also to properly clean it, before switching to open-water operations. Relying on a latest-generation wireline cement logging tool to provide A-annulus initial and final pictures, on CT to circulate the tubing and benefit from the induced reservoir thermal effect, and on a remotely operated vehicle (ROV) to lubricate A-annulus proved successful in achieving the objectives. Additionally, the logging tool clearly mapped oil presence in the A-annulus, whose displacement was thus, properly anticipated and performed without interruption. During Phase 2, the well was successfully temporarily and then definitively abandoned as per plan. The complexity of the intervention was overcome safely thanks to a multidisciplinary approach, and the lessons learned can be applied in the industry for P&A operations.