The move came after nearly three years of to-ing and fro-ing between HP and the former management of Autonomy, the software company which was a darling of the UK tech scene before it was acquired by HP in 2011 for $11bn (£7.4bn). Months later, an argument over Autonomy's accounting practices caused HP to take an £8.8bn writedown on it, kicking off one of the most epic wars of words in modern business history.
But wait. Wasn't HP already suing Lynch? Or Lynch suing HP? And wasn't there something going on with Deloitte? And what about the Serious Fraud Office?
Here's what's going on
Although various insults have been traded, until this week the only lawsuit that took place was at the behest of HP's shareholders, who filed a class action suit against the company in 2013, accusing HP of ignoring evidence that Autonomy was "vastly overvalued".
Although several settlements had been submitted, until this month they were rejected by a US judge. To make matters more complicated, former Autonomy chief financial officer Hussain twice sought to block settlements, in one arguing that through the settlements HP management would "never answer for their mismanagement of the Autonomy acquisition and their own securities fraud".
However, earlier this month a preliminary settlement was approved. A final hearing is due in July.
It's not clear who made the first move this week, but what is clear is that within hours of one another, HP and Team Lynch/Hussain had, respectively, filed a claim in the High Court and threatened to file a claim in UK courts.
What about everyone else?
At various points in the process, outsiders have waded in. Britain's Serious Fraud Office disclosed in 2013 that it was investigating the sale of Autonomy to HP. But in January this year it handed control of the investigation to the US authorities, saying there was "insufficient evidence for a realistic prospect of conviction".
The other outsider is Deloitte. In August last year reports appeared saying HP was planning to sue the "big four" accounting company, which acted as Autonomy's auditor. But the company has not been named as a defendant in any lawsuit surrounding the company.