On his return to the service in March 1907 Mulock joined the torpedo course at HMS "Vernon". Shortly before Scott had invited Mulock to join him on a second polar expedition; the young lieutenant accepted without hesitation. Then Ernest Shackleton wrote to him inviting him to participate in the Nimrod Expedition. Due to the gentleman's agreement with Scott, Mulock declined but unintentionally sparked off a rivalry by revealing to Shackleton that Scott was planning to "go south again"; it came as no surprise that in April 1908 Mulock failed to qualify as a torpedo lieutenant. Commodore J. Briggs, RN, commanding "Vernon", recorded on Mulock's official service record that "his failure to qualify in torpedo was due to his inattention", a clear rebuke for the efforts of Scott and Shackleton. In 1908 Mulock was appointed to (Captain Henry Oliver, RN) in preparation for a command of his own.