Valkyrie I housed the Royal Navy's No. 1 Radar Training School. Ratings billeted in guest houses requisitioned for the duration along Loch Promenade, then walked up to the training buildings on Douglas Head, including the Douglas Head Hotel. One of those buildings is now occupied by Manx Radio. The early work in radar required serious investment: £3 million in 1941 money, equivalent to roughly £147 million by 2018. To teach radar operators what to do when an aircraft showed up on the scope, the school had a single Sea Hurricane on call from 772 Squadron of the Fleet Air Arm, based at RAF Andreas at the northern tip of the island. The aircraft made simulated attacks for the students to track. In October 1944 it crashed on Douglas Head, killing the pilot, Sub Lieutenant Robert Paton.

While the school operated, Douglas Head was closed to the public. The popular Marine Drive that ran out to Port Soderick, a favourite Edwardian and pre-war excursion, was off limits. The cliffs that had been a recreational area in summer were now a sensitive military site. For visiting tourists in 1939 it had been a place to walk and take photographs of the sea; for the duration it was a place where the Navy's most secret electronic technology was rehearsed against an aircraft pretending to attack.

In September 1943, Valkyrie II was commissioned to train signalmen and wireless telegraphy ratings for the landing craft being assembled for D-Day. Existing establishments could not produce enough personnel in time, so men were transferred from the Army and the RAF and put through a six-month course. At peak, 2,500 ratings were under training at once. The catering problem of feeding so many men was solved by cooking naval supplies at the Empress and Palace hotels, where civilian contractors ran the kitchens. The Palace Dance Hall was turned into a mess hall where 1,200 ratings could sit down at once. A section of Douglas Beach was fenced off so the training, much of which had to happen outdoors, could proceed away from civilians. By March 1944, the work of Valkyrie II was finished. The invasion of France was about to begin, and the establishment was quietly paid off.

HMS Valkyrie trained radar operators from across the Allied navies. From June 1943 onwards, more than 400 French sailors were trained at Douglas under the command of Lieutenant Jean Colin, a detachment that arrived at the start of the commission and stayed through much of the war. Among the British personnel who served at Valkyrie was a young man named Jon Pertwee, decades before he would become the third Doctor Who. While stationed on the island, Pertwee helped form an amateur dramatic society called the Service Players, of which he was made an honorary life member. The Navy was producing radar operators; Douglas was producing a future television star almost as a side effect.

At 16:00 on Monday 30 December 1946, the White Ensign at HMS Valkyrie was lowered for the last time. The Douglas Promenade premises were vacated immediately, and the Douglas Head buildings followed on 31 January 1947. Over the lifetime of the establishment, more than 30,000 naval ratings had passed through the Royal Navy's No. 1 Radar Training School. The guest houses returned to civilian use. The Palace Dance Hall went back to dances. Marine Drive reopened. The radar sets and the Sea Hurricane and the secrecy moved on to other places. What remained in Douglas was a memory of an extraordinary expansion, when a seaside resort had been converted into one of the largest training schools the Royal Navy ever ran on shore.