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Mycroft reveals to Sherlock and Watson that he is British Intelligence, an asset to MI6. When Sherlock is propositioned by Myrcoft's handler to investigate the murder of a former MI6 analyst, he accepts.
When the murder victim is discovered armless at the morgue, it is revealed that his arms contained invisible ink that made up a code to reveal the mole within the MI6.
After Joan turns down Mycroft's offer for a romantic relationship, she discovered he rejoined British Intelligence to protect Sherlock. This act causes her to fall for Mycroft and they sleep together.
Sherlock then discovers that Mycroft is being framed.