Once in London, Roelfzema became involved in a secret service group of escaped young Dutchmen called the Mews, after Chester Square Mews, the neighborhood in London where they lived. The group was being run under the direction of François van 't Sant, director of the Dutch Military Intelligence and Security Service (CID, ). The agents were trained under Colonel Euan Rabagliati of MI9. Their operation was called 'Contact Holland', whose goal was to establish links with the disparate Dutch resistance groups, support them and coordinate their activities in the Netherlands. Most agents were brought into the Netherlands by parachute drop. This required the agent to be trained in parachuting and exposed them to the risks of doing so at night. Roelfzema, Peter Tazelaar and Chris Krediet came up with an alternative method of insertion. They proposed gaining access to the Netherlands by sea. They would use a Motor Gun Boat (MGB) to cross the channel, covering the last stretch of water by taking a small boat and rowing into shore.