The use of a companion app to augment viewing of information-rich television programmes is investigated. The app displays a synchronised graphical abstraction of a programme's content in the form of a concept map. Two experiments were conducted involving participants watching an astronomy documentary with the app. The first compared watching the programme with and without the app, the second compared non-interactive and interactive versions of the app. Understanding of the programme, cross-device gaze behaviour and user experience of the app were assessed. Our results show that the companion app improved participants' understanding and recall of the programme. Participants were found to manage their visual attention systematically when using the companion app and correlations were found in the way they shifted their gaze from TV screen to tablet and back in response to changes in the programme content. Increasing interaction with the app disrupted understanding of the television programme and visual attention. Participants were positive about the value of companion apps for understanding and recall of programmes but distraction and 'knowing where to look' were significant concerns.